Larval fish assemblage in the Baía River (Mato Grosso do Sul State,Brazil): temporal and spatial patterns

Synopsis We sampled the Baía River (Mato Grosso do Sul State, Brazil) monthly, to survey taxonomic composition and temporal and spatial distribution of fish larvae. The ichthyoplankton was mainly composed by larvae of small and medium sized sedentary species and it was numerically dominated by six taxa: Plagioscion squamosissimus, Hypophthalmus edentatus, Hoplias aff. malabaricus, Bryconamericus stramineus, Serrasalmus spp. and Catathyridium jenynsii. These taxa present distinct temporal and spatial occurrence patterns: H. edentatus, B. stramineus and C. jenynsii are abundant between September and March in lentic areas; H. aff. malabaricus and Serrasalmus spp. were caught between October and February in lotic areas; whereas P. squamosissimus occurs in all sampled areas, with peak of capture in January. Hypophthalmus edentatus, H. aff. malabaricus, Serrasalmus spp. and B. stramineus were the taxa that most contributed to structure the assemblages temporally and spatially, and their abundances were influenced by the interaction of several environmental variables.     

Influence of ontogenetic changes in prey selection on the survival and growth of rohu, Labeo rohita and singhi, Heteropneustes fossilis larvae

When offered a mixed diet of different zooplanktonic items covering a body size range of 75–2200 μm, (a) rohu, Labeo rohita and (b) singhi, Heteropneustes fossilis larvae ingested progressively larger prey as they grew, due to age-related increase in gape. However, a nearly constant prey size/mouth size ratio was maintained for a period of 4wk after hatching. The dominance of rotifers in the diet during the first 2-wk was followed by cladocerans, particularly Moina macrocopa. Significant differences observed in the growth rates of the larvae reared on different diet regimes were related to ontogenetic changes in prey selection. An exclusive copepod diet throughout resulted in the lowest weight gain in the larvae of both species. However, copepods had no apparent adverse effects when present with the preferred rotifers and cladocerans. Although constituting a suboptimal prey size for the older larvae, rotifers alone, when present in sufficient densities, produced growth rates comparable to those obtained on a cladoceran diet. However, a mixed diet regime contributed to the maximum growth. The implications of these findings to rearing larvae of the economically important rohu and singhi are discussed.     

Diel and seasonal patterns of food intake and prey selection by Coregonus sp. in re-oligotrophicated Lake Lucerne, Switzerland

Feeding intensity by whitefish Coregonus sp., in oligotrophic Lake Lucerne in Switzerland was high during dusk when the bulk of potential prey items were in the depth zone occupied by the fish. Diet composition was fairly uniform throughout the day but changed substantially over the seasons. The fish fed opportunistically; differences between seasons reflected changes in prey availability. During the intensive feeding and growing period (May-September), fish were found in the upper 20 m of the lake feeding primarily on cladocerans. Large and non-evasive species, Daphnia spp. and Bythotrephes longimanus , were the most numerous and frequent organisms in the diet during the major part of the growing season. Smaller ( Bosmina spp.) as well as evasive species (cyclopoid copepods) were consumed in large numbers when larger, non-evasive species were rare in the lake. The fish showed strong preference for the least abundant crustacean, B. longimanus , while the most abundant crustaceans, calanoid copepods, were rare in the diet. The fish not only selected particular species but, within each species, selected the larger individuals. Diel vertical migration of the prey items in this lake could be, at least in part, attributed to fish predation pressure. The observed selectivity patterns shown by the fish are explained in terms of prey visibility, escape ability, the overlap in distribution of predators and prey in time and space, the profitability of the prey and the present trophic state of the lake.     

Feeding ecology of larval and juvenile black seabream (Acanthopagrus schlegeli) and Japanese seaperch (Lateolabrax japonicus) in Tolo Harbour, Hong Kong

Black seabream, Acanthopagrus schlegeli, and Japanese seaperch, Lateolabrax japonicus, are important commercial species in the coastal waters of western Pacific Ocean, including Japan, Korea and China. In Hong Kong, larvae and juveniles of these two species occur in bays and estuaries during late winter and spring. This study reports on the ontogenetic changes in food habits in larvae and juveniles of these species in an artificial rocky shore area. Copepods and cladocerans were the most numerous food items for black seabream. There was a shift to larger and benthic prey as the fishes grew. Japanese seaperch <2.1cm fed predominantly on copepods and cladocerans, while larger prey were added as fish size increased. Japanese seaperch >6.0cm were piscivorous. Maximum prey width increased with fish standard length and mouth gape width in both species. Overall, black seabream showed greater diet breadth than did Japanese seaperch. In black seabream, diet breadth increased with fish size. In Japanese seaperch, diet breadth increased with size for fishes <4.0cm, then decreased as the fishes became piscivorous. Prey selectivity in black seabream was determined using information on prey availability in plankton samples. In general, preference was stronger for cypris larvae, Penilia avirostris and decapod larvae than for copepods and podonids. In recent years, overfishing and environmental degradation have led to the decline of fish populations in Tolo Harbour. Absence of fishes with empty gut indicates that inner Tolo Harbour is still an important nursery area for these two commercial species.     

Diet composition,feed preferences and mouth morphology of early stage silver therapon (Leiopotherapon plumbeus,Kner 1864) larvae reared in outdoor tanks

This study examined the diet composition, feeding preferences, and mouth morphology of the silver therapon (Leiopotherapon plumbeus, Kner 1864) larvae under captive conditions. Larvae were reared in outdoor tanks (4 m³) with natural food grown 2 weeks prior to start of larval rearing. Food preference was measured by the Chesson's electivity index (α_i). Gut content analysis of larvae sampled between 5 and 25 days after hatching (DAH) showed the dominance in the diet by zooplankton, mainly copepod nauplii, cladocerans and insect larvae. Small fish larvae (5–9 DAH; 3.32–6.29 mm standard length) preferred cladocerans, ciliates and copepod nauplii; whereas older larvae (12–25 DAH; 5.45–19.26 mm standard length) preferred insect larvae over cladocerans and adult insects. The mouth gape size at 5 DAH was 359 μm and increased to 3.75 mm at 40 DAH when body size grew at an average rate of 0.59 mm d^?1. The standard length (SL) of L. plumbeus larvae was strongly associated with mouth size (r² = 0.98, P < 0.05), indicating a progressive increase of ingested prey size of the fish larvae. These results clarified the early life feeding ecology of this species, which is essential in developing effective hatchery techniques.     

Effect of habitat complexity on the predation of Buenoa fuscipennis (Heteroptera: Notonectidae) on mosquito immature stages and alternative prey

Notonectids are well‐known predators in aquatic habitats, where mosquito larvae, chironomids, and cladocerans constitute their main diet. Our purpose was to assess the effect of structural complexity on the predatory ability of Buenoa fuscipennis, a common predator in aquatic habitats of Buenos Aires city (Argentina). Buenoa fuscipennis showed type 2 functional responses in both the presence and absence of prey refuge and no differences in attack rate or handling time between refuge treatments. Regarding mosquito size classes, B. fuscipennis exhibited a significantly higher preference for 2^nd instar larvae and no predation on pupae. In the presence of mosquito larvae and alternative prey, B. fuscipennis preferred mosquitoes over chironomid larvae and adult cladocerans over mosquito larvae. No switching behavior was detected in our experiments. Habitat structure only slightly affected the predator´s consumption rates on mosquito larvae. Overall, preference for prey did not vary with the presence of refuge, except for the preference for mosquitoes over chironomid larvae, which was significantly decreased in the presence of refuge as a consequence of reduced predation on mosquito larvae. The results suggest that B. fuscipennis could efficiently control mosquitoes in structurally simple habitats where chironomids are the most abundant alternative prey but not in temporary pools where cladocerans are abundant.     

Cladoceran Densities,Day-to-Day Variability in Food Selection by Smelt,and the Birth-Rate-Compensation Hypothesis

Late-evening gut inspection of a dominant planktivore (smelt) and evaluation of densities, fecundities, and body-size distributions in dominant zooplankton prey (cladocerans) were made in day-to-day sequences in June–July (24 days in 1999 and 24 days in 2000). This was conducted as a field test of the hypothesis that species-specific population densities in cladocerans result from size-selective predation by a dominant fish assumed to be a general predator, switching from one prey to another as relative abundance changes. Little of the expected coincidence has been revealed between population density declines and increased numbers of a given prey in smelt diet. However, the data were consistent with the notion that fish would switch from one prey to another depending on the prey relative abundance (the number of prey a fish would see in its reaction field volume). Each cladoceran population fluctuated around its species-specific density level, lower or higher, depending on individual susceptibility to smelt predation, from 0.2 ind. l^–1 in large-bodied Daphnia hyalina and Leptodora kindtii, to 30.0 ind. l^–1 in small-bodied Daphnia cucullata andBosmina thersites. In spite of high fish-to-fish and day-to-day variability in both smelt diet and smelt selectivity for different prey, all cladocerans (also copepods and midge larvae) were equally persistent in smelt diet, and smelt selectivity was similar for small- and large-bodied prey categories, but lower for elongated-(Daphnia, Diaphanosoma) than for compact-body (Bosmina, Chydorus) species, when integrated for the entire sampling time. Closer examination of D. cucullata and B. thersites revealed strong smelt selection for later instars and females with greater clutches, showing that size distribution in a cladoceran population might be structured by fish predation in a similar way to that a cladoceran community (species relative abundance) is structured in a lake habitat. The birth-rate-compensation hypothesis is offered to explain why the value of food selectivity index in a planktivorous fish would remain the same for alternate prey categories with similar life-history traits, unless they differ in susceptibility to predation before the time of first reproduction.     

Feeding habits and diet overlap of marine fish larvae from the peri-Antarctic Magellan region

The Magellan region is a unique peri-Antarctic ecosystem due to its geographical position. However, the knowledge about the distribution and feeding ecology of fish larvae is scarce. Since this area is characterized by low phytoplankton biomass, we hypothesize that marine fish larvae display different foraging tactics in order to reduce diet overlap. During austral spring 2009–2010, two oceanographic cruises were carried out along southern Patagonia (50–56°S). Larval fish distribution and feeding of the two most widely distributed species were studied, the smelt Bathylagichthys parini (Bathylagidae) and black southern cod Patagonotothen tessellata (Nototheniidae). Larvae of B. parini showed a lower increase in the mouth gape at size, primarily feeding during daytime (higher feeding incidence during the day) mostly on nonmotile prey (invertebrate and copepod eggs, appendicularian fecal pellets, diatoms). They showed no increase in feeding success (number, total volume of prey per gut and prey width) with increasing larval size, and the niche breadth was independent of larval size. Larvae of P. tessellata showed a large mouth gape at size, which may partially explain the predation on motile prey like large calanoid copepods (C. simillimus) and copepodites. They are nocturnal feeders (higher feeding incidence during night) and are exclusively carnivorous, feeding on larger prey as the larvae grow. Nonetheless, niche breadth was independent of larval size. Diet overlap was important only in individuals with smaller mouth gape (<890 μm) and diminished as larvae (and correspondingly their jaw) grow. In conclusion, in the peri-Antarctic Magellan region, fish larvae of two species display different foraging tactics, reducing their trophic overlap throughout their development.     

Predation on Mosquito Larvae by Mesocyclops thermocyclopoides (Copepoda: Cyclopoida) in the Presence of Alternate Prey

The cyclopoid copepod Mesocyclops thermocyclopoides, a dominant invertebrate predator in many shallow ponds and temporary water bodies in northern India, feeds on cladocerans, rotifers, ciliates and when present, on mosquito larvae also. We studied in the laboratory the prey consumption rates of the copepod on first and fourth instar larvae of two species of mosquito (Anopheles stephensi and Culex quinquefasciatus) in relation to their density. We also studied its prey selectivity with mosquito larvae in the presence of an alternate prey (the cladocerans‐either Moina macrocopa or Ceriodaphnia cornuta) in different proportions. With either mosquito species, the copepod actively selected Instar‐I larvae, avoiding the Instar‐IV larvae, and with either instar, selected Anopheles stephensi over Culex quinquefasciatus. When prey choice included the cladoceran as an alternate prey, the copepod selected the cladoceran only when the other prey was Instar‐IV mosquito larvae. Our results point to the potential and promise of M. thermocyclopoides as a biological agent for controlling larval populations of vectorially important mosquito species.     

Comparative parasitism of the fish Plagioscion squamosissimus in native and invaded river basins

Biological invasions are considered a major threat to biodiversity around the world, but the role of parasites in this process is still little investigated. Here, we compared parasite infections of a host species in the areas where it originated and where it was introduced, and in native and introduced species in the same environment, using the endoparasites of the fish Plagioscion squamosissimus (Sciaenidae) in 3 Brazilian basins. Samples were taken in 2 rivers where the species is native, i.e., Solim?es River (SO) and Tocantins River (TO), and where the species was introduced, the upper Paraná River (PR). In addition, abundances of diplostomids and larval nematodes were compared between P. squamosissimus and 2 native competitors in the PR, Hoplias malabaricus and Raphiodon vulpinus. In total, 13 species of endoparasites were recorded, but only Austrodiplostomum sp. and cestode cysts were present in all localities. Although infracommunity richness was similar, their species composition was slightly different among localities. General linear models using the relative condition factor of fish as response variables, and abundance of the most prevalent parasites as possible predictors showed that the condition of fish is negatively correlated with parasite abundance only in the native range (TO). Abundance of diplostomid eye flukes was higher in the PR, and in the native species H. malabaricus when compared to the invader, which might present an advantage for P. squamosissimus if they compete for prey. However, although P. squamosissimus may have lost some of its native parasites during its introduction to the PR, it is now possibly acting as a host for native generalist parasites.     

The effect of invertebrate diets on lipids, fatty acid composition and physiological condition of pike larvae (Esox lucius)

The present study tests two invertebrate diets for pike larvae reared in floating cages. One diet contained mainly copepods and cladocerans such as Eucyclops serrulatus and Bosmina longirostris , while the other diet contained Eucyclops serrulatus together with a small proportion of large Chydoridae and chironomid larvae. During the first days of the experiment, the amount of food distributed to the larvae was insufficient (9–17.1 prey per larva and per day) and led to an increase in the mortality. Triacylglycerol contents of fry were low and dietary fatty acids were either catabolised or, concerning PUFA such as 22:6(n-3), incorporated into phospholipids. From day 13 to the end of the study (day 22), the most abundant diets distributed were accompanied by an increase in triacylglycerol PUFA and in triacylglycerol contents of larvae. During the same period weight and length growth were better for larvae reared on the copepod plus Chydoridae and chironomid diet, than for larvae reared on the copepod and Bosmina diet. The use of a lipid condition index based on the triacylglycerol/sterol ratio suggested that Chydoridae and chironomids positively influenced the growth and nutritional condition of larvae. The effects of prey type in terms of PUFA composition on pike larvae are discussed.     

The food and feeding relationships of larval and 0+ year juvenile fishes in lowland rivers and connected waterbodies. II. Prey selection and the influence of gape

The relative importance of taxa- and size-specific prey selection, and the influence of gape on the prey consumed by the larvae and 0+ year juveniles of four fish species were investigated in 'main river', 'marina' and 'pond' macrohabitats in the lower River Trent, England. A general sequence of ontogenetic shifts in food consumption was reflected in the electivity indices of particular prey taxa, partly due to the restrictions imposed by the gape of 0+ year fishes. Certain taxa, however, were consistently selected over others, irrespective of size, suggesting that taxa-specific, as well as size-specific, prey characteristics may be important in the selection process. There were significant, positive relationships between maximum prey (zooplankton) length and maximum gape height for larvae, but not for 0+ year juveniles. The majority of fishes, however, consumed prey substantially smaller than the maximum theoretically possible inferred from their gape. The greater size ranges of zooplankton in connected waterbodies compared with main river channels provide suitable prey for a range of developmental steps and fish species, and may, thus, enhance recruitment success.     

Length–weight relationships of eight fish species from the lower section of the Uruguay River (Río Negro, Uruguay)

This study reports length–weight relationships for Apareiodon affinis , Leporinus obtusidens , Cynopotamus argenteus , Loricariichthys melanocheilus , Parapimelodus valenciennis , Iheringichthys labrosus , Rhinodoras dorbignyi and Pachyurus bonariensis . Specimens were collected between 2006 and 2007 from the lower section of the Uruguay River. The study represents the first reference on weight–length relationships for three species and also provides the new maximum size for three species.     

Long-term responses of zooplankton to invasion by a planktivorous fish in a subarctic watercourse

1. Introduced or invading predators may have strong impacts on prey populations of the recipient community mediated by direct and indirect interactions. The long-term progression of predation effects, covering the invasion and establishment phase of alien predators, however, has rarely been documented.
2. This paper documents the impact of an invasive, specialized planktivorous fish on its prey in a subarctic watercourse. Potential predation effects on the crustacean plankton, at the community, population and individual levels, were explored in a long-term study following the invasion by vendace ( Coregonus albula ).
3. Over the 12-year period, the density and species richness of zooplankton decreased, smaller species became more abundant and Daphnia longispina , one of the largest cladocerans, was eliminated from the zooplankton community.
4. Within the dominant cladocerans, including Daphnia spp., Bosmina longispina and Bosmina longirostris , the body size of ovigerous females and the size at first reproduction decreased after the arrival of the new predator. The clutch sizes of Daphnia spp. and B. longirostris also increased.
5. Increased predation pressure following the vendace invasion induced many effects on the crustacean zooplankton, and we document comprehensive and strong direct and indirect long-term impacts of an introduced non-native predator on the native prey community.     

Prey selection by the larvae of three species of littoral fishes on natural zooplankton assemblages

The prey selection of larvae of three common littoral fish species (pike, Esox lucius; roach,Rutilus rutilus; and three-spined stickleback, Gasterosteus aculeatus) was studied experimentally in the laboratory by using natural zooplankton assemblages. Zooplankton prey was offered at four different concentrations to study the functional responses of the planktivores. The diets of pike and sticklebacks were formed mainly of copepod juveniles and adults, which dominated the prey communities, although sticklebacks ate also cladocerans. The diet of roach larvae consisted of rotifers, cladocerans and copepods, without prey selection, in equal proportions indicating a more omnivorous diet. All fish larvae were able to feed selectively although in sticklebacks prey selection was less pronounced. Pike and roach larvae preferred large prey to smaller prey types. Patterns of prey selection are discussed in the context of size-selection theory and apparent vs. true selection.     

Size of prey ingested by whitefish,Coregonus sp., larvae. Are Coregonus larvae gape-limited predators?

The feeding ecology of whitefish, Coregonus , larvae in oligotrophic Lake Sarnen and eutrophic Lake Hallwil is presented, taking into consideration the size of the prey ingested in relation to the mouth size of larvae and to the availability of zooplankton.
When larvae grow from 10 to 16 mm their mouth width increases from 500 to 1000 μm while their gape height, with the mouth open at 45 and 90°, increases from 200 to 400 μm and from 400 to 700 μm, respectively, whichever lake is considered.
The differences found in the mean prey width between the two lakes arise in part from the composition of diets, and in part from differences in size of the available prey organisms. Whatever the origin of the larvae and the taxon we consider, larvae seem to ingest the most abundant prey size without selecting the biggest ones available even if, from a mechanical point of view, they would be able to ingest them.
Taking into account the generally low density of zooplankton in the uppermost layer of the lake, we conclude that whitefish larvae eat in the manner such as to decrease the cost of prey capture, in terms of energy and time, by choosing the more abundant prey available.     

Diet reveals links between morphology and foraging in a cryptic temperate reef fish

Predators select prey so as to maximize energy and minimize manipulation time. In order to reduce prey detection and handling time, individuals must actively select their foraging space (microhabitat) and populations exhibit morphologies that are best suited for capturing locally available prey. We explored how variation in diet correlates with habitat type, and how these factors influence key morphological structures (mouth gape, eye diameter, fin length, fin area, and pectoral fin ratio) in a common microcarnivorous cryptic reef fish species, the triplefin Helcogrammoides cunninghami. In a mensurative experiment carried out at six kelp‐dominated sites, we observed considerable differences in diet along 400 km of the Chilean coast coincident with variation in habitat availability and prey distributions. Triplefins preferred a single prey type (bivalves or barnacles) at northern sites, coincident with a low diversity of foraging habitats. In contrast, southern sites presented varied and heterogeneous habitats, where triplefin diets were more diverse and included amphipods, decapods, and cumaceans. Allometry‐corrected results indicated that some morphological structures were consistently correlated with different prey items. Specifically, large mouth gape was associated with the capture of highly mobile prey such as decapods, while small mouth gape was more associated with cumaceans and copepods. In contrast, triplefins that capture sessile prey such as hydroids tend to have larger eyes. Therefore, morphological structures co‐vary with habitat selection and prey usage in this species. Our study shows how an abundant generalist reef fish exhibits variable feeding morphologies in response to the distribution of potential habitats and prey throughout its range.     

Ecological redundancy between coral reef sharks and predatory teleosts

Reef sharks may be ecologically redundant, such that other mesopredatory fishes compensate for their functions when they decline in number, preventing trophic cascades. Oral jaw gape, hereafter referred to as gape, determines maximum prey size in many piscivores and therefore affects the size structure of prey assemblages. Here, we examine whether gape and maximum prey size differ between five species of reef shark and 21 species of teleost (n?=?754) using data collected from 38 reefs in the Indo-Pacific. Sharks displayed relatively small gape dimensions compared to most teleost species and, at smaller sizes, the giant trevally Caranx ignobilis and other teleosts may be able to consume larger prey than similar-sized sharks. However, ecological redundancy between reef sharks and teleosts appears to decline at larger sizes, such that the grey reef shark Carcharhinus amblyrhynchos, for example, may be capable of consuming larger prey than any other reef predator at its largest sizes, regardless of prey body shape. Moreover, sharks may be able to consume proportionally larger prey as they grow, in contrast to reef teleosts, which may largely be limited by their gapes to ever-smaller prey as a proportion of their body size. Our results also suggest that reef sharks may be unable to swallow whole prey that are >?36% of their length, consistent with gut-content studies. Conservation of reef ecological function may therefore depend not only on the protection of sharks but also particular size classes and key components of the mesopredatory guild.

     

Functional morphology of gills and respiratory area of two active rheophilic fish species, Plagioscion squamosissimus and Prochilodus scrofa

Measurements of gill dimensions were carried out on two ecologically distinct active rheophilic teleost species, the curvina Plagioscion squamosissimus and the curimbatá Prochilodus scrofa , and were analysed in relation to body mass according to the equation Y=aW^b . The gill respiratory areas of P. squamosissimus and P. scrofa were large as expected for active fish and increased with increasing body mass ( b =0.70 and 0.72, respectively) showing no significant difference between them. However, the large respiratory area of both species was realized in a different way revealing an adaptation to the plasticity of head components related to feeding habits. Consequently, significant differences were found between the number and average length of gill filaments and the bilateral area of secondary lamellae. The respiratory area of P. scrofa was due mainly to larger bilateral surface area of the secondary lamellae and its growth coefficient ( b =0.51) that was significantly higher ( P <0.05) than that found for P. squamosissimus ( b = 0.36). The frequency of secondary lamellae mm⁻¹ of filament was similar in both species (22 ± 2 on one side of gill filament). The dimensions of gill components and the respiratory area of these species suggest a complex interaction between head form, and feeding habits related to the functional morphology of the gills to meet the oxygen requirements of each species.     

A Review of Predation Impact by 0+ Fish on Zooplankton in Fresh and Brackish Waters of the Temperate Northern Hemisphere

To assess potential differences in predation impact on zooplankton communities by small (larva) and large 0+ juvenile fish, 18 studies were reviewed from fresh water and the brackish Baltic Sea of the northern hemisphere temperate region. These case studies were performed either in the field or in mesocosm experiments. Larva stocks were found to exert only minor impact on small zooplankton species such as rotifers, copepodids and small cladocerans. In contrast, stocks of 0+ juveniles were found to have the potential to depress populations of large cladocerans and copepods, especially during late summer and autumn. However, studies where both 0+ juvenile fish consumption and zooplankton dynamics and production were exactly quantified are still very rare, and therefore final evaluation of this interaction cannot be made. In addition, papers were summarized that describe differences in morphological and physiological performance between larva and 0+ juvenile fish. The greater impact of 0+ juvenile fish on large zooplankton may be explained by their larger mouth gape and by their better developed abilities to detect and consume their prey items. However, this partly is lessened by the lower energy requirements of juvenile fish compared with identical biomasses of fish larvae, although larva bioenergetics remains only fragmentarily understood. Consequently, selective predation by fish larvae on particular small zooplankton prey may be more important than has been detected so far.